Introduction of oxygen into soil and groundwater for the purpose of removing or neutralizing contaminants is known. Some of those known means include the following:
In in-situ air sparging, a surface-mounted air compressor is used to force atmospheric air into the subsurface below the depth of the water table. The bubbling of air through the water in sparging wells acts to dissolve oxygen into the groundwater and transfer contaminants to air. The disadvantage of this system is that it has high capital costs for installation and maintenance of compressors, noise, and limited effectiveness by transferring contaminants to another media. Additionally, air is mostly nitrogen, so relatively less oxygen is exposed to groundwater for dissolution.
Oxygen sparging is similar to air sparging, except that a small blower is used and pure oxygen replaces air forced into the subsurface. The disadvantages of this system are similar to air sparging, except oxygen exposure is increased. However, cost of oxygen gas cylinders is added to operation and maintenance.
In another system, a bag of solid material that releases oxygen at a slow constant rate upon contact with groundwater acts to increase the content of oxygen in the aquifer. Upon exhaustion of the material, it is replaced with another bag of oxygen-releasing material. The disadvantage of this system is that relatively low amounts of oxygen are introduced compared to other technologies. Furthermore, the oxygen content is only increased in water that comes within close proximity to the bag of material. Accordingly, this application is not appropriate at sites where quick treatment is needed.
In another system, pure oxygen is passed down a tube to a cartridge submerged in groundwater. A series of long porous Teflon (R) tubes containing this oxygen are exposed to the groundwater and oxygen dissolves into the groundwater through diffusion and direct dissolution. A disadvantage with this system is that oxygen distribution to the aquifer is limited by the diffusion and dissolution rate for the groundwater and environmental conditions such as temperature and pH. Additionally, there is no transport mechanism as part of this application, so movement of oxygenated water is limited by the hydraulic gradient of the groundwater.
In another system, electrolysis of the groundwater itself is used to add oxygen. Electrolysis is the dissociation of water into component ions of hydrogen and oxygen. The reaction is represented at each respective electrode by:
Anode (oxidization)2H2O→O2(g)+4H++4e−
Cathode (reduction)4H2O+4e−→2H2(g)+4OH−
Electrolysis promotes active remediation mechanisms in groundwater contaminated with petroleum hydrocarbons (or other organic compounds) by creating hydrogen and oxygen ions to carry electrical current across the circuit, and forming hydrogen and oxygen atoms that combine to form molecular H2 and O2 gas. Hydrogen gas is sparingly soluble in water and most of it escapes to the vapor phase. Oxygen is more soluble, and as the gas forms, some immediately dissolves in the groundwater, increasing the dissolved oxygen content. This dissolved oxygen provides a means by which bacteria can break down petroleum hydrocarbons in groundwater. The scale of this affect is within the well bore, and outside the well bore as far as the oxygen demand for chemical and biological sources is satisfied.
In addition to increasing oxygen content, the electrolysis process changes the oxidation/reduction potential (ORP) in groundwater to favor the oxidation of chemical species occurring in the groundwater and aquifer materials in contact with water. This has the effect of improving the potential for breakdown of organic contaminants dissolved in groundwater. The scale of this effect can extend to a significant distance outside the well bore by water transport. As ORP increases, chemical oxygen demand decreases due to decreasing reduced mineral concentrations (e.g., ferrous to ferric iron).
In addition, in the electrolytic process, hydroxyl radicals are formed between the electrodes in an intermediate reaction. Petroleum hydrocarbons and other organic compounds passing between the electrodes while they are energized can be broken down to carbon dioxide and water by chemical oxidation. The scale of this effect is within the electrolytic cell only.
In one prior art electrolysis system, a longitudinally mounted electrode pair is employed and groundwater is circulated multiple times between solid plate electrodes and a storage tank until the desired concentration of dissolved oxygen is reached. The water is then passed out into the aquifer. The disadvantage of this system is that the longitudinal mounting restricts the possible flow rate through the cell, requiring multiple passes. The power applied is limited due to the electrochemical properties of the electrodes, which limits the amount of oxygen that can be generated at any given time. Additionally, if precipitation or other clogging occurs within the cell, flow could be interrupted. The potential for such clogging is increased due to the number of times the water must flow past the electrodes.